Spray a 12″ skillet with cooking spray, and heat over medium-high heat. Add sausage, and sauté and crumble until cooked through.
Pour in tomatoes, marinara sauce, and water. (Watch out for splattering in this step. You can remove the skillet from the heat and let it cool slightly before pouring in the tomatoes to avoid a mess.) Bring to a rapid simmer.
Break lasagna noodles into small pieces and stir into the skillet. There should be just enough liquid to cover the noodles. Add up to 1/2 c. more water if needed. Cook at a low boil for the recommended cooking time on your noodles, approximately 15 minutes.
Reduce heat to medium low. Stir in spinach a couple handfuls at a time until slightly wilted. Stir in ricotta cheese, and return to a simmer. Spread shredded cheese over the top, turn off heat, and cover pan for 5-10 minutes to allow cheese to melt and sauce to thicken.
You can use frozen spinach instead if you prefer. Just thaw it and squeeze it dry before adding it to the lasagna.